If the input-side DC link of a DC/DC converter on the primary side of a high-frequency transformer is charged by, for example, an increasing output voltage from a connected photovoltaic module, which normally involves charging at least one buffer capacitor in said DC link, this will have no effect on the charging state of the output-side DC link of the DC/DC converter on the secondary side of the high-frequency transformer. When the switches of an inverter bridge used to supply current to the primary side of the high-frequency transformer from the input-side DC link start to get alternately closed, extremely high currents will result due a missing counter-voltage in the output-side DC link. These currents can easily exceed a maximum threshold above which the integrity of the DC/DC converter and, in particular, its semiconductor components are jeopardized. This holds especially if the switches of the inverter bridge are operated at a frequency in the order of a magnitude of the resonant frequency of a resonant circuit including the primary side of the high-frequency transformer in order to minimize the passive losses in the DC/DC converter. In this case, the currents that flow when the switches of the inverter bridge are closed during DC/DC converter startup are almost entirely unattenuated. An intentional attenuation of these currents would in return increase power loss under normal DC/DC converter operation.
A known solution to this problem involves operating the switches of the inverter bridge during startup at a frequency that is significantly higher than a frequency at which the switches are operated during normal operation of the DC/DC converter. The increased frequency used in this method depends on the voltage present at the input-side DC link. The higher the voltage is the higher is the frequency at which the inverter bridge switches are operated during startup. As a consequence of increasing the frequency while keeping the time in which the switches are open constant (as a percentage value), currents can increase only for a very short period of time before their direction is reversed by the reversal of switch positions. This means that increasing the frequency when operating the switches of the inverter bridge during startup has the direct effect of limiting the currents. Note, however, that this measure is insufficient if the output-side DC link of the DC/DC converter is not yet charged at all. Therefore, internal power supply transformers of described prior art feature an additional winding that pre-charges the output-side DC link to 150 V so that the voltage difference between the input-side DC link and the output-side DC link is reduced in advance. Both the additional frequency for clocking the inverter bridge switches and the additional winding in the internal power supply transformer mean a significant amount of effort in order to effectively limit the currents when starting up the DC/DC converter with a high-frequency transformer.
U.S. Pat. No. 6,091,610 A discloses a system and a method for reducing transient switch currents in an asymmetrical half bridge converter. The converter includes a high-frequency transformer having a split DC link and an asymmetrical inverter half bridge comprising a main switch and a complementary switch at its primary side and a rectifier half-bridge as well as a DC link at its secondary side. A soft-start of the converter is described as one way to avoid transient currents capable of causing switch failure. Initially, the asymmetrical half bridge is off and the duty cycle of the main switch is at zero. To turn on the asymmetrical half bridge, the duty cycle of the main switch is gradually increased until the steady state duty cycle is reached. The output voltage may thus be gradually increased, thereby avoiding high peak currents. However, since the main and complementary switches are on for complementary periods, the complementary switch may initially be on for a large period of time. Energy stored in a second input capacitor may, therefore, rapidly discharge through the complementary switch during the initial switching cycles. This rapid discharge may produce a large pulse of current through the complementary switch, causing it to get damaged. To reduce the current stress occurring in the complementary switch as the converter turns on, a conductive path is coupled across the DC link at the primary side of the transformer to substantially discharge it and thereby reduce the current stress in the complementary switch when the converter turns on.
Thus, there still is a need for a method and a DC/DC converter in which the maximum currents present during startup are effectively limited without needing to fulfill complicated equipment requirements to this end.